This appendix covers additional material and details relevant to Chapter 5 of this report. These pages address several topics related to processes used in the manufacturing of ceramics for armors and include discussions on potential armor materials such as functionally graded materials, biomimetics, foams, smart sensors, and phononic band gap materials.

The pressureless sintering process offers cost reduction through net shape processing using innovative powders and processing methods to obtain full density without the application of pressure. The goal of densification is to create strong bonds in the material and eliminate porosity so that theoretical densities, along with homogeneous microstructures, can be achieved for the sintered bodies. Residual porosity, along with the shape and size distributions of the pores and grains, influences the characteristics of dynamic performance. The low-cost alumina armor plates manufactured by means of uniaxial pressing, slip casting, and sintering are used in vehicle armor applications in large volumes. The typical cost of these plates runs about $2.50/lb to $10/lb for the finished tile. Solid state sintering is achieved by heating the “green” compact to the temperature that is in the range between approximately 50 and 80 percent of the melting temperature.1

At these temperatures the powder does not melt, but fusing of adjacent powder particles and reduction in the overall porosity occur by atomic diffusion in the solid state. Solid state sintering is typically used for pure, single-phase polycrystalline materials, such as α–Al2O3. For many covalently bonded polycrystalline ceramics, the required dense microstructure is hard to achieve using solid state sintering; therefore, additives are used to form a small amount of liquid phase between the particles at the sintering temperature. The liquid phase provides a high-diffusivity path to transport matter into the pores and facilitates densification. For example, hard-to-sinter silicon carbide (SiC) is processed as a liquid-phase sintered ceramic. The price of SiC tiles manufactured by means of a pressureless sintered process is in the range of $40/lb to $50/lb.

Solid- and liquid-state sintering processes are widely employed to densify refractory ceramics, but at much higher temperatures than required by the hot-pressing (HP) technique. Sintering of SiC was first performed by Prochazka,2 using boron and carbon as sintering aids to reduce the interfacial energy of the grains (boron),3 and by reacting the carbon with residual silica (carbon) present on the SiC particle surface.4,5Sintering of b-SiC is more difficult than sintering of α–SiC because of the b toα phase transformation at 1900°C to 2000°C, which generates voids between grains owing to the difference in the growth morphology of b and α grains.6 Several additives, such as Al–C, Al2O3–C, and Al2O3–Y2O3, have been tested as sintering aids for SiC powder to enhance the sintering rate and to reduce grain

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Appendix E
Processing Techniques and Available
Classes of Armor Ceramics
This appendix covers additional material and details rel- lently bonded polycrystalline ceramics, the required dense
evant to Chapter 5 of this report. These pages address several microstructure is hard to achieve using solid state sintering;
topics related to processes used in the manufacturing of ce- therefore, additives are used to form a small amount of liq-
ramics for armors and include discussions on potential armor uid phase between the particles at the sintering temperature.
materials such as functionally graded materials, biomimetics, The liquid phase provides a high-diffusivity path to trans-
foams, smart sensors, and phononic band gap materials. port matter into the pores and facilitates densification. For
Some of the key manufacturing processes, together with example, hard-to-sinter silicon carbide (SiC) is processed
their advantages and disadvantages, are listed in Table 5-1 as a liquid-phase sintered ceramic. The price of SiC tiles
of Chapter 5. Table E-1 presents the relevant properties of manufactured by means of a pressureless sintered process is
materials listed in Table 5-1 of Chapter 5. in the range of $40/lb to $50/lb.
Solid- and liquid-state sintering processes are widely
employed to densify refractory ceramics, but at much higher
PRESSURELESS SINTERING
temperatures than required by the hot-pressing (HP) tech-
nique. Sintering of SiC was first performed by Prochazka,2
The pressureless sintering process offers cost reduction
through net shape processing using innovative powders and using boron and carbon as sintering aids to reduce the
interfacial energy of the grains (boron),3 and by reacting
processing methods to obtain full density without the ap-
plication of pressure. The goal of densification is to create the carbon with residual silica (carbon) present on the SiC
particle surface.4,5 Sintering of b-SiC is more difficult than
strong bonds in the material and eliminate porosity so that
sintering of α–SiC because of the b to α phase transforma-
theoretical densities, along with homogeneous microstruc-
tures, can be achieved for the sintered bodies. Residual tion at 1900°C to 2000°C, which generates voids between
porosity, along with the shape and size distributions of the grains owing to the difference in the growth morphology of
b and α grains.6 Several additives, such as Al–C, Al2O3–C,
pores and grains, influences the characteristics of dynamic
performance. The low-cost alumina armor plates manufac- and Al2O3–Y2O3, have been tested as sintering aids for SiC
tured by means of uniaxial pressing, slip casting, and sinter- powder to enhance the sintering rate and to reduce grain
ing are used in vehicle armor applications in large volumes.
The typical cost of these plates runs about $2.50/lb to $10/
lb for the finished tile. Solid state sintering is achieved by
heating the “green” compact to the temperature that is in 2Prochazka, S. 1974. Hot pressed silicon carbide. U.S. Patent 3,853,566,
the range between approximately 50 and 80 percent of the filed December 21, 1972, and issued December 10, 1974, to General Electric
melting temperature.1 Company. Schenectady, NY.
3Maddrell, E. 1987. Pressureless sintering of silicon carbide. Journal of
At these temperatures the powder does not melt, but
Materials Science Letters 6(4): 486-488.
fusing of adjacent powder particles and reduction in the 4van Rijswijk, W., and D. Shanefield. 1990. Effects of carbon as a sinter-
overall porosity occur by atomic diffusion in the solid state. ing aid in silicon carbide. Journal of the American Ceramic Society 73(1):
Solid state sintering is typically used for pure, single-phase 148-149.
polycrystalline materials, such as α–Al2O3. For many cova- 5Hamminger, R. 1989. Carbon inclusions in sintered silicon carbide.
Journal of the American Ceramic Society 72(9): 1741-1744.
6Williams, R., B. Juterbock, C. Peters, T. Whalen, and A. Heuer. 1984.
Forming and sintering behavior of B- and C-doped α- and b-SiC. Journal
1Rahaman, M. 2007. Sintering of Ceramics. Boca Raton, Fla.: Taylor
and Francis Group. of the American Ceramics Society 67(4): C62-C64.
125

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127
APPENDIX E
Compared to lower-density, pressureless sintered prod-
raw material cost and ease of fabrication. Large-size alumina
ucts, the hot-pressing process is a slow batch process that
panels up to 400 mm × 550 mm are currently produced by
means of pressureless sintering14 for use in lightweight armor typically yields a close-to-full-density product with superior
ballistic properties. Hot-pressed SiC and B4C provide supe-
vehicles and police car door protection. Morgan is one is the
rior ballistic properties but are manufactured at high cost
main producers of armor-grade alumina made by the pres-
and in limited volumes through a batch process. The U.S.
sureless sintering technique.
Army Manufacturing Technology Program has developed
an HP apparatus with multiple heating and cooling chambers
HOT PRESSING
and central hot pressing chamber to reduce the cost of hot
pressed 4 in. × 4 in. tiles from $135/lb to $85/lb.16 The goal
Hot pressing is often the procedure of choice for the
is to increase production volumes, thus reducing the cost to
manufacture of opaque ceramics, since it can produce fully
$35/lb, similar to pressureless sintered material.
dense ceramics at reasonably moderate temperatures and
Due to processing differences, hot-pressed and pressure-
pressures. However, HP can only produce limited shapes
less sintered materials have different microstructures; see,
such as flat plates or those with a small curvature. A current
for example, Figure E-1. Hot-pressed material is typically
Army program is developing HP to fabricate SiC tiles at
lower cost.15 fully dense with fine microstructures, whereas pressureless
sintered materials have large grains with texture. However,
Traditional hot pressing is a batch process in which the
recent dynamic magnetic compaction (DMC) work has
“green” powder compacts are formed by means of a suitable
shown the promise of obtaining fine grain structures similar
pressing method and then loaded into a hot-pressing die.
to those of HP material by combining DMC and pressure-
Some armor manufacturers use tape casting or extrusion to
less sintering. Such a process needs to be further developed.
build up green B4C armor shapes for hot pressing. The die
and powder are ramped up to the sintering temperature and
pressure is applied to the die. To meet the required high sin-
CURRENT-ASSISTED SINTERING
tering temperatures (>2000°C), heated dies made of graphite
Nano-grain-size ceramic powders are currently being
or other high-strength inert materials are used in special
explored by various laboratories, including the U.S. Army
hot-pressing furnaces. As the ceramic part size increases,
Research Laboratory, to obtain better mechanical proper-
the load requirements increase, making the HP equipment
ties through microstructural modifications. However, when
large. HP is associated with small production volumes, and
ceramic powders are hot-pressed or sintered at very high
typically large billets are produced to be cut into individual
temperatures for extended times, grain growth takes place.
armor tiles. Often, multiple parts, separated by spacers,
Processing methods such as dynamic compaction or spark
are pressed together to increase production rate. Once the
plasma sintering (SPS) techniques can be used to retain the
ceramic is hot-pressed, it is cooled and then machined to its
small grain size of nanograined powders.
final dimensions by diamond tools using slow machining
One method of accelerating the sintering process of
steps and grinding rates.
difficult-to-sinter armor ceramics involves the use of electri-
Materials with different final densities and mechanical
cal current. The name most often used for such field-assisted
properties are produced by varying the nature of powder
sintering is SPS, but the process is also known as plasma
additives and hot-pressing conditions such as pressure,
pressure sintering, pulsed electric current, and electric-
temperature, and time. Typically, in hot-pressing SiC armor
material, powders of α–SiC are mixed with suitable sinter- pulse-assisted consolidation. Significant advantages exist
for using current-assisted sintering over that of hot-pressing,
ing aids (boron and carbon, for example); additional carbon
hot isostatic pressing, or pressureless sintering; the most im-
is added to remove the silica passivation layers from the
portant advantage is lower sintering temperature and reduced
SiC particles. Temperature steps are adjusted based on the
holding time, which results in marked comparative improve-
specific mix of powders and additives, and during the last
ments in mechanical properties.17 For example, attaining a
high-temperature cycle, pressure is applied to achieve maxi-
heating rate of 600°C/min, typically used in SPS of ceramics,
mum densification. Different final density and mechanical
could help retain the homogeneous grain size distribution
properties are reached by varying the hot-pressing condi-
tions and the powder additives used. Additives and precise
processing conditions are kept as proprietary information by
different manufacturers. 16Campbell, J., J. LaSalvia, W. Roy, E. Chin, R. Palicka, and D. Ashkin.
2008. New Low-Cost Manufacturing Methods to Produce Silicon Carbide
14Medvedovski, E. 2010. Ballistic performance of armour ceramics: (SiC) for Lightweight Armor Systems. ADA504013. Proceedings of the
Influence of design and structure Part 2. Ceramics International 36(7): Army Science Conference (26th). Accessed April 4, 2011.
17Munir, Z., U. Anselmi-Tamburini, and M. Ohyanagi. 2006. The effect
2117-2127.
15Protection materials—Research to acquisition. Briefing by E. Chin, of electric field and pressure on the synthesis and consolidation of materi -
U.S. Army Research, Development and Engineering Command, to the als: A review of the spark plasma sintering method. Journal of Materials
committee, July 23, 2010. Science 41(3): 763-777.

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128 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
FIGURE E-1 Silicon carbide sample microstructures showing grains in (left) hot-pressing, (center) dynamic magnetic compaction followed
by pressureless sintering, and (right) uniaxial pressing followed by pressureless sintering. SOURCE: Chelluri, B., and E.A. Knoth. 2008.
SiC armor tiles via magnetic compaction and pressureless sintering. Presentation to the 32nd International Conference and Exposition on
Advanced Ceramics and Composites, January 26-February 1, Daytona Beach, Florida.
along with the small average grain size of B4C.18,19 The over the past decade.21,22,23,24 In reaction bonding, which
spall strengths20 of SPS-processed B4C and SiC ceramics uses silicon-based matrixes, the pressureless infiltration of
are improved over those from hot-pressing or pressureless a powder preform is achieved by good wetting and a highly
exothermic reaction between liquid silicon and carbon.25 The
sintering techniques. SPS is still under development, espe-
cially for sintering larger parts. Density variations are still process is known variously as reaction bonding, reaction
observed in the case of difficult-to-sinter ceramics because sintering, self-bonding, or melt infiltration.
of current flow along the highest-conductivity graphite die M Cubed Technologies Inc., a developer of the reac-
walls, although low or no current density is detected inside tion bonding process, uses a process that includes the
the part. As a result of such a temperature gradient, material following steps: (1) mixing of B4C (or SiC) powder and a
density varies with its location relative to the die walls. The binder to make a slurry; (2) shaping of the slurry by various
widespread adoption of SPS in the past decade was pos- techniques, such as casting, injection molding, pressing,
sible because of the availability of commercially built SPS and others; (3) drying and carbonizing of the binder; (4)
systems. Currently the two major players are SPS SYNTEX green machining; (5) infiltration (reaction bonding) with
Inc., Japan, and FCT Systeme GmbH, Germany. Recently, molten Si (or alloy) above 1410°C in an inert or vacuum
the U.S. firm Thermal Technology LLC also started selling atmosphere; and (6) solidification and cooling. During the
field-assisted sintering furnaces. However, to the best of the infiltration step, carbon in the preform reacts with molten
committee’s knowledge, there are no commercially produced
21Waggoner, W., B. Rossing, M. Richmond, M. Aghajanian, and A. Mc-
ceramic armor tiles using SPS, suggesting a potential oppor-
Cormick. 2003. Silicon carbide composites and methods for making same.
tunity for improved processing of dense ceramics.
U.S. Patent 6,503,572, filed July 21, 2000, and issued January 7, 2003, to
M Cubed Technologies, Monroe, Conn.
22Aghajanian, M., B. Morgan, J. Singh, J. Mears, and R. Wolffe. 2002.
REACTION-BONDED CERAMICS
A new family of reaction bonded ceramics for armor applications. Ceramic
Transactions 134: 527-539.
Reaction-bonded SiC and reaction-bonded boron car-
23Aghajanian, M., McCormick, B. Morgan, and A. Liszkiewicz, Jr. 2005.
bide have been successfully used for armor applications
Boron carbide composite bodies, and methods for making same. U.S. Patent
6,862,970, filed November 20, 2001, and issued March 8, 2005, to M Cubed
Technologies. Monroe, Conn.
24Karandikar, P., M. Aghajanian, and B. Morgan. 2003. Complex, NET-
18Hayun, S., S. Kalabukhov, V. Ezersky, M. Dariel, and N. Frage. 2010. shape composite components for structural, lithography, mirror and armor
Microstructural characterization of spark plasma sintered boron carbide applications. Pp. 561-566 in 27th Annual Cocoa Beach Conference on
ceramics. Ceramics International 36(2): 451-457. Advanced Ceramics and Composites: B. Ceramic Engineering and Sci-
19Hayun, S., V. Paris, M.P. Dariel, N. Frage, and E. Zaretzky. 2009. ence Proceedings 24(4). W. Kriven and H-T. Lin, eds. Hoboken, N.J.: John
Static and dynamic mechanical properties of boron carbide processed by Wiley & Sons.
25Karandikar, P., S. Wong, G. Evans, and M. Aghajanian. 2010. Micro -
spark plasma sintering. Journal of the European Ceramics Society 29(16):
3395-3400. structural development and phase changes in reaction bonded boron carbide.
20Paris, V., N. Frage, M., Dariel, and E. Zaretsky. 2010. The spall strength Pp. 5-22 in Advances in Ceramic Armor VI: Ceramic Engineering and Sci-
of silicon carbide and boron carbide ceramics processed by spark plasma ence Proceedings 31(5). Swab, J., S. Mathur, and T. Ohji, eds. Hoboken,
sintering. International Journal of Impact Engineering 37(11): 1092-1099. N.J.: John Wiley & Sons.

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130 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
crystalline surface coatings, with grain sizes as small as a few serves to toughen by many of the same mechanisms that oc-
tens of nanometers, can be synthesized by means of electro- cur in natural nacre. The entire array is fastened onto the steel
deposition, thermal spray, sputter deposition, metal-organic plates of tanks and similar vehicles using Velcro. The armor
chemical vapor deposition, and eletrophoretic deposition.47 has been implemented onto various ground and air vehicles,
Many of these processes can create surface layers wherein including 1,000 High Mobility Multipurpose Wheeled Ve-
the grain sizes are smoothly graded from the surface to the hicles (Humvees) for the U.S. Marines.
bulk, bringing about controlled gradients in strength and Another example of a natural material for biologically
fracture toughness. Similarly, improved resistance to contact inspired design is the skins of certain fish. Both the scales and
damage can be achieved by tailoring gradients in porosity dermis of fish skin are highly pliant, lightweight, and resist
penetration—all of this in an ultrathin structure.51 Although
below the contact surfaces. Current materials synthesis and
processing capabilities, engineered gradations in proper- fish scales have received very little attention, it is recognized
ties—from nanometer to macroscopic length scales—appear that bony scales are difficult to penetrate and dissipate en-
ergy quite well.52 Scales form a physical barrier that deters
promising for the design of improved fracture-, damage-, and
wear-resistant structures and surfaces and for armor protec- attacks by predators; indeed, they were likely the inspiration
tion applications. for scale armor not only in ancient times but also in modern
times.53 While its hierarchical organization is important for
the overall mechanical performance of fish skin,54 the contri-
BIOMIMETIC MATERIALS
butions at the different length scales are rarely investigated.
Natural materials that are mechanically robust often It is not known, for instance, how adjacent scales interact to
have hierarchical designs. Abalone nacre, rat teeth, fish thwart penetration, but this mechanism should be understood
scales, wood, and spider silk exhibit highly complex hierar- if its performance in the new generation of ultralight pliant
chical structures, multifunctionality, and even self-healing armor systems is to be replicated.
capabilities48 and thus are appealing to mimic for use in Biomorphic ceramics using natural products such as
advanced armor design. The remarkable mechanical perfor- wood and cellulose-fiber paper and cardboard have also been
of interest for their potential use as armor materials.55 In
mance of certain natural materials stems from their complex
ordered microstructure, organized over several length scales, particular, wood-based biomorphic SiC (bioSiC) is a promis-
even though the materials are often made of relatively weak ing material for armor. The fabrication of bioSiC entails the
constituents. Nacre, which is found in a number of mollusk rapid mineralization of wood, during which the wood is car-
shells, combines stiffness and strength along with a high bonized and then infiltrated with either Si vapor or Si melt.
level of toughness. Mimicking of the abalone structure was The formed SiC replicates the wood microstructure, and the
first attempted in the 1980s, when a laminated structure of diversity of the wood texture results in a large and varied
Al–B4C was produced.49 A significant increase in fracture selection of bioSiC ceramics. Novel biomorphic SiC ceram-
toughness (up to 16 MPa-m 1/2) was achieved; however, ics have been successfully developed at DLR, Germany’s
national research center for aeronautics and space.56,57 At
Al4C3 was formed during the processing of the laminates,
limiting the useful armor application of the produced lami- DLR, wood-based preforms are converted to SiSiC materials
nates owing to low hardness and strength and the high brittle- using the liquid silicon infiltration process. In this process, a
ness of the Al4C3 phase. Another example of mimicking green body or preform based on low-cost raw materials—for
abalone nacre, but on a macroscale, was conducted a decade
ago by Foster-Miller50—LAST (Light Applique Segmented 51Vernerey, F., and F. Barthelat. 2010. On the mechanics of fishscale
Tile) armor plates were produced using Coors alumina and structures. International Journal of Solids and Structures 47(17): 2268-2275.
SiC or B4C hexagonal tiles covered in a thermoset laminate 52Bruet, B., J. Song, M. Boyce, and C. Ortiz. 2008. Materials design
of Kevlar and held together with a Velcro-type adhesive. A principles of ancient fish armour. Nature Materials 7: 748-756.
53Vernerey, F., and F. Barthelat. 2010. On the mechanics of fishscale
nacre-like armor structure such as this absorbs energy and
structures. International Journal of Solids and Structures 47(17): 2268-2275.
54Fratzl, P., and R. Weinkamer. 2007. Nature’s hierarchical materials.
47Suresh, S. 2001. Graded materials for resistance to contact deformation Progress in Materials Science 52(177): 1263-1334.
55Medvedovski, E. 2010. Ballistic performance of armour ceramics:
and damage. Science Magazine 292(5526):2447-2451. Available online at
http://www.sciencemag.org/content/292/5526/2447.full. Accessed October, Influence of design and structure. Part 1. Ceramics International 36(7):
11, 2010. 21032115.
48Meyers, M., P-U. Chen, A. Lin, and Y. Seki. 2008. Biological materi - 56Heidenreich, B., M. Gahr, E. Strassburger, and E. Lutz. 2010. Biomor-
als: Structure and mechanical properties. Progress in Materials Science phic SiSiC-materials for lightweight armour. Pp. 21-33 in Proceedings of
53(1): 1-206. 30th International Conference on Advanced Ceramics & Composites 2010.
49Sarikaya, M., and I.A. Aksay. 1992. Nacre of abalone shell: A natural Hoboken, N.J.: John Wiley & Sons.
57Heidenreich, B., M. Crippa, H. Voggenreiter, H. Gedon, M. Nordmann,
multifunctional nanolaminated ceramic polymer composite material. Pp.
1-25 in Structure, Cellular Synthesis and Assembly of Biopolymers (Re- and E. Strassburger. 2010. Development of biomorphic SiSiC- and C/
sults and Problems in Cell Differentiation) 19(1). S. Case, ed. Amsterdam: SiSiC- materials for lightweight armour. Advances in Ceramics Armor VI:
Springer-Verlag. Ceramic Engineering and Science Proceedings (31). Hoboken, N.J.: John
50QinetiQ. www.foster-miller.com. Accessed October 13, 2010. Wiley & Sons.

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131
APPENDIX E
consists of b-SiC formed as a result of the interaction of
example, wood fibers and phenolic resin—is manufactured
by means of warm pressing. One preform used for manu- Si and C, a significant quantity of residual Si (up to 30 vol
facturing biomorphic SiSiC is medium-density fiberboard, percent), and a very small amount of carbon (up to 3 vol
which is widely used in the furniture industry. The preform percent). The typical structure of bioSiC is more homoge-
is made by pressing fine fibers of needle wood with binders neous than conventional reaction-bonded SiC and is defined
based on formaldehyde or phenolic resins in a mass produc- by the particular wooden preform. The typical size of SiC
tion process, making very large panels—typically 1.22 m × grains is 5 to 20 μ, but owing to the relatively high content of
residual Si, the density is about 2.8 g/cm3. BioSiC ceramics,
2.44 m (4 ft × 8 ft) up to 2.8 m × 6.5 m (9.2 ft × 21.3 ft)—at a
cost of about $1.75/kg. After pyrolysis, the porous C-preform such as one manufactured using inexpensive preforms from
medium-density fiberboard66 can be produced inexpensively
is siliconized in a vacuum at temperatures above 1450°C;
next, capillary forces allow molten silicon to infiltrate the for armor systems consisting of large, single-piece compo-
open pores of the C-preform. The resulting reaction with nents. Biomorphic siliconized silicon carbide (SiSiC) has
the carbon forms SiSiCx.58 The final composition—that is, demonstrated good potential for use in lightweight ceramic
the content of SiC, Si, and C—is heavily influenced by the armor systems. Although manufacturing defects and exces-
porosity and microstructure of the C-preform and can be sive residual silicon in bioSiC reduce ballistic performance,
varied widely by using tailored green bodies. Because practi- especially in multi-hit situations, appropriate armor system
cally no change in geometry occurs during siliconization or design—that is, with the right selection of ceramic thickness
in reproducible contraction rates during pyrolysis, even large and type and backing thickness—allows the materials to
and complicated shaped parts can be manufactured using a withstand even armor-piercing rounds. In order to increase
cost-effective, near-net-shape technique. resistance to multiple hits, novel materials based on the
The processing technology for bioSiC makes the pro- combination of biomorphic SiSiC and C/C-SiC ceramics
duction of complex shapes relatively easy; production is have been developed, with carbon fibers integrated into bio-
much cheaper, because the bioSiC forms at much lower morphic SiSiC to increase ductility and damage tolerance.
temperatures than those required for SiC sintering or hot- There is special interest in replicating dragline silk,
pressing techniques. Biomorphic SiC shows excellent ther- the extremely strong silk that forms the framing threads of
momechanical performance along with structural stability spider webs. The comparative properties of selected silk and
over a wide range of temperatures.59,60,61,62,63,64,65 BioSiC manmade fibers are presented in Table E-2.
Mimicking the structure of materials found in nature
might provide insight into the creation of armor materials
58Gahr, M., J. Schmidt, W. Krenkel, A. Hofenauer, and O. Treusch. 2004.
with superior ballistic properties. However, the task of pro-
Dense SiSiC ceramics derived from different wood-based composites:
ducing manmade materials with similar microstructures and
processing, microstructure and properties. P. 425 in Proceedings of the 5th
performance is challenging precisely because the structures
International Conference on High Temperature Ceramic Matrix Composites.
Westerville, Ohio: The American Ceramic Society. are so complex.
59Martinez-Fernández, J., F. Valera-Feria, and M. Singh. 2000. High tem-
perature compressive mechanical behavior of biomorphic silicon carbide
MACHINING, GRINDING, AND POLISHING CERAMICS
ceramics. Scripta Materialia 43(9): 813-818.
60Martínez-Fernández J., F. Valera-Feria, Rodríguez, A., and M. Singh.
The machining, grinding, and polishing of ceramics are
2000. Microstructure and thermomechanical characterization of biomorphic
silicon carbide-based ceramics. Pp. 733-740 in Environment Conscious expensive processes. For example, the final shape of the ar-
Materials: Ecomaterials. 39th Annual Conference of Metallurgists. Ottawa, mor product from a flat, hot-pressed part is created by grind-
Canada: Canadian Institute of Mining.
ing with diamond wheels. The pressureless sintered process,
61Singh, M., J. Martínez-Fernández, A., and de Arellano-López. 2003.
with its much larger shrinkage (20 to 30 percent), requires
Environmentally conscious ceramics (ecoceramics) from natural wood pre -
considerably more grinding to achieve final tolerances.
cursors. Current Opinion in Solid State and Materials Science 7(3): 247-254.
62de Arellano-López, A, J. Martínez-Fernández, P. González, C. Domín -
Therefore, grinding and finishing costs make the final cost
guez, V. Fernández-Quero, and M. Singh. 2004. Biomorphic SiC: A new of hot-pressed parts higher than that of pressureless sintered
engineering ceramic material. International Journal of Applied Ceramic
parts. Part geometry and concentricity or parallelism also
Technology 1(1): 56-67.
affect the final cost. For example, improving the tolerance of
63Varela-Feria, F., J. Martínez-Fernández, A. de Arellano-López, and M.
the outside diameter from 0.020 in. to 0.010 in. can double
Singh. 2002. Low density biomorphic silicon carbide: Microstructure and
mechanical properties. Journal of the European Ceramic Society 22(14-15): the cost of a piece. Typically, a ground part has a surface
2719-2725. finish tolerance of 16 μin. or better. A better finish of 4 μin.
64Varela-Feria, F., J. Ramírez-Rico, A. de Arellano-López, J. Martínez-
can be obtained using lapping and honing, but will cost more.
Fernández, and M. Singh. 2008. Reaction-formation mechanisms and
microstructure evolution of biomorphic SiC. Journal of Materials Science
43(3): 933-941.
65Bautista, M., A. de Arellano-López, J. Martínez-Fernández, A. Bravo- 66Heidenreich, B., M. Gahr, and E. Medvedovski. 2005. Biomorphic reac-
Léon, and J. López-Cepero. 2009. Optimization of the fabrication process tion bonded silicon carbide ceramics for armor applications. Pp. 45-53 in
for medium density fiberboard (MDF)-based biomimetic SiC. International Proceedings of the 107th Annual Meeting of the American Ceramic Society.
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132 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
TABLE E-2 Tensile Mechanical Properties of Spider Silks and Other Materials
Toughness (MJ-m–3)
Material Stiffness (GPa) Strength (GPa) Strain to Failure
Natural fibers
Araneus major Ampullate (MA) silk 10 1.1 0.27 160
A. viscid silk 0.003 0.5 2.7 150
40-130a
Nephila clavipes silk 11-13 0.88-0.97 0.17-0.18
208b
N. edulis silk
Bombyx mori cocoon silk 7 0.6 0.18 70
35-55c
B. mori silk (w/sericin) 5-12 0.5 0.19
30-70d
B. mori silk (w/o sericin) 15-17 0.61-0.69 0.4-0.16
Synthetic materials
Nylon fiber 5 0.95 0.18 80
Kevlar 49 fiber 130 3.6 0.027 50
Carbon fiber 300 4 0.013 25
High-tensile steel 200 1.5 0.008 6
aGosline, J., M. DeMont, and M. Denny. 1986. The structure and properties of spider silk. Endeavour 10(1): 37-43; Zemlin, J. 1968. A study of the mechani -
cal behavior of spider silks. Technical Report 69-29-CM (AD 684333). U.S. Army Natick Laboratory, Natick, Mass.; Cunniff, P., S. Fossey, M. Auerbach, J.
Song, D. Kaplan, W. Adams, R. Eby, D. Mahoney, and D. Vezie. 1994. Mechanical and thermal properties of dragline silk from the spider Nephila clavipes.
Polymers for Advanced Technologies 5(8): 401-410.
bVollrath, F., B. Madsen, and Z. Shao. 2001. The effect of spinning conditions on the mechanics of a spider’s dragline silk. Proceedings of the Royal Society
268(1483): 2339-2346.
cPérez-Rigueiro, J., C. Viney, J. Llorca, and M. Elices. 1998. Silkworm silk as an engineering material. Journal of Applied Polymer Science 70(12):
2439-2447.
dPérez-Rigueiro, J., C. Viney, J., Llorca, and M. Elices. 2000. Mechanical properties of single-brin silkworm silk. Journal of Applied Polymer Science
75(10): 1270-1277.
FOAMS thicknesses, using 20 mm fragment simulating projectiles
(FSP). A test performed with an impact velocity of 1,067 m/s
Foam is a complex assemblage of dispersed voids or
on a baseline setup, followed by a test on the baseline with
pores separated by a film. The reason for using foams to
a 12.7-mm-thick piece of aluminum foam incorporated into
absorb shock wave energy is that as the shock wave passes
the material stack, revealed that the rise time of the stress
into the foam, the individual cells collapse; it is through this
wave increased from 1 ms for the baseline sample to 2 ms
deformation that energy is absorbed. Foams can be made
for the sample with foam. The use of foam also delayed the
from any number of materials and may be open- or closed-
time for the stress wave to reach the stress gauge by about
cell, but it is the metal foams, particularly aluminum, and
14.6 ms. The maximum stress reached in both cases was
the polymeric foams, particularly polyurethane, that are em -
about 6.25 GPa. The air-filled cellular structure of the foam
ployed most frequently in shock wave research.67,68,69 Two
is not conducive to wave propagation because the waves are
important directions for future research are (1) constructing
only transmitted along the cell walls, which, owing to their
foams from a wider variety of materials and (2) developing
random orientations, tend to disperse the wave. For a foam
methods for greater control over foam microstructure. Such
thickness of 12.7 mm, a stress of 0.825 GPa was recorded in
model foams will help computational efforts on porous
the ceramic tile, while a thickness of 30.48 mm completely
structures.
eliminated the stress recorded in the tile. In this case, the
The use of foams as impact barriers was demonstrated
foam was not fully compacted by the FSP and thus acted as
by Gama et al.,70 who performed impact tests on layered
an excellent wave barrier.
composite armor systems with various foam positions and
The location of the foam is also important, demon-
strating the need not only to consider the inherent material
67Hanssen, A., L. Enstock, and M. Langseth. 2002. Close-range blast
properties in isolation but also to consider them as part of
loading of aluminum foam panels. International Journal of Impact Engi -
the overall armor system.
neering 27(6): 593-618.
68Ramachandra, S., P. Sudheer Kumar, and U. Ramamurty. 2003. Impact
energy absorption in an Al foam at low velocities. Scripta Materialia 49(8):
TRANSPARENT CERAMICS AND EMBEDDED DAMAGE
741-745.
SENSORS
69Gama, B., T. Bogetti, B. Fink, C. Yu, T. Dennis Claar, H. Eifert, and J.
Gillespie. 2001. Aluminum foam integral armor: A new dimension in armor
Transparent armor ceramics must provide good trans-
design. Composite Structures 52(3-4): 381-395.
parency in the visible (0.4-0.7 μ) and mid-infrared (1-5 μ)
70Ibid.

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133
APPENDIX E
optical ranges and protection against fragmentation from have potential for use in new dome construction requiring
ballistic impacts, with multi-hit capability and minimal dis- substantial durability and high transparency across the opera-
tortion around the impacted regions.71,72 tional bandwidth for infrared-guided missile sensing.79,80,81
As mentioned in Chapter 5, transparent armor systems Special glasses and glass-ceramics such as lithium disilicate
are typically constructed of multiple layers of armor-grade or aluminum-lithium-based crystallized or partially crystal-
ceramic plates, separated by transparent polymer (for ex- lized structures also offer advantages as host materials for
ample, polycarbonate) interlayers, and are bonded together laser use, since their refractive indexes and strain-optical
with a transparent adhesive. The polymer phase mitigates coefficients can be readily controlled through changes in
chemical composition.82
the stresses generated by thermal expansion mismatches and
inhibits crack propagation from ceramic to polymer. Poly-
meric materials such as transparent nylons, polyurethanes,
Damage-Reactive Sensors for Armor
and acrylics have also been explored as separators, but they
have not been widely used in armor protection owing to less- Combat vehicles could be outfitted with smart ceramic
than-optimal optical and durability characteristics. Transpar- sensors built into the protective armor material. Such sen-
ent alumina (Al2O3) and magnesia (MgO) are two commonly sor could detect and report on structural damage in real
used transparent ceramic armor materials. The composite time. Structural damage caused by a wide range of ballistic
system formed by these two materials provides good protec- impacts can be expected to affect armor structures under
tion against high-velocity ballistic projectiles.73,74 Silicon battlefield conditions. Changes in the armor’s structural
nitride (Si3N4), a nonoxide ceramic, has also been employed condition can be detected by tiny piezoelectric transducers,
for use in radomes because of its good transit of microwave or sensors, built into the protective armor plate material.
energy and its superior mechanical strength. Piezoelectric sensors are usually ferroelectric, perovskite
These materials can be produced as transparent poly- structure materials—for example, lead zirconate/lead tita-
crystalline ceramic parts, often with complex geometries, by nate, barium titanate, and others—that have been suitably
using standard ceramic-forming techniques such as pressing, doped and electrically poled to optimize their piezoelectric
(hot) isostatic pressing, and slip casting.75 response characteristics.83,84
Nanocomposite ceramic materials of yttria (Y2O3) and Given the above relationships, the piezoelectric trans-
magnesia (MgO) have been explored for use in transparent ducers can be designed both to generate and to receive volt-
armor protection. The materials exhibit an average grain size age responses when coupled with ultrasonic waves that are
of approximately 200 nm, and near-theoretical transmission generated to pass through the material. To determine the best
in the 3 to 5 μ infrared band range. These complex ceramic response characteristics for particular environmental condi-
n anocomposites reportedly offer improved mechanical tions, the ultrasonic signals may vary over a wide frequency
properties such as superplastic flow and metal-like machin- range (1.0 kHz to 200 kHz). A generated shock wave through
ability. However, mechanical failure modes and armor pro- the plate picks up the reflections of sound waves and con-
tection characteristics must still be fully evaluated for these verts them into electrical voltages from which, with suitable
nanocomposite materials systems.76,77,78 These materials amplification, one can determine their spectra and whether
the plate is cracked or damaged.
71Patel, P., G. Gilde, P. Dehmer, and J. McCauley. 2000. Transparent
ceramics for armor and EM window applications. P. 1 in Proceedings of the
International Society for Optics and Photonics 4102(1).
72Harris, D. 2009. Materials for infrared windows and domes: Proper- 79Huang, Z., X. Sun, Z. Xiu, S. Chen, and C-T. Tsai. 2004. Precipitation
ties and performance. Bellingham, Wash.: International Society of Optical synthesis and sintering of yttria nanopowders. Materials Letters 58(15):
Engineers. 2137-2142.
73Villalobos, G., J. Sanghera, and I. Aggarwal. 2005. Transparent ce - 80Jeong, J., S. Park, D. Moon, and W. Kim. 2010. Synthesis of Y O nano-
23
ramics: Magnesium aluminate spinel. Naval Research Laboratory Optical powders by precipitation method using various precipitants and preparation
Sciences Division. of high stability dispersion for backlight unit (BLU). Journal of Industrial
74Hogan, P., R. Stefanik, C. Willingham, and R. Gentilman. 2004. Engineering Chemistry 16(2): 243-250.
81Nihara, K., and T. Sekino. 1993. New nanocomposite structural ceram -
Transparent yttria for IR windows and domes—Past and present. Raytheon
Integrated Defense Systems. ics. P. 405 in Materials Research Society Symposium Proceedings held at
75Ibid. the Nanophase and Nanocomposite Materials Symposium. Warrendale, Pa.:
76Bisson, J-F., Lu Jianren, K. Takaichi, Yan Feng, M. Tokurakawa, A. Materials Research Society.
82Hartmann, P., R. Jedamzik, S. Reichel, and B. Schreder. 2010. Optical
Shirakawa, A. Kaminskii, H. Yagi, T. Yanagitani, and K-I. Ueda. 2004.
Nanotechnology is stirring up solid-state laser fabrication technology. glass and glass ceramic historical aspects and recent developments: A Schott
Recent Research Developments in Applied Physics 7(Part II): 475-469. view. Applied Optics 49(16): D157-D176.
77Wen, L., X. Sun, S. Chen, and C-I. Tsai. 2003. Synthesis of nanocrystal- 83Meitzler, T., G. Smith, M. Charbeneau, E. Sohn, M. Bienkowski, I.
line yttria powder and fabrication of transparent YAG ceramics. Journal of Wong, and A. Meitzler. 2008. Crack detection in armor plates using ultra-
the European Ceramics Society 24(9): 2681-2688. sonic techniques. Materials Evaluation 66(6): 555-559.
78Wen, L., X. Sun, Q. Lu, G. Xu, and X. Hu. 2006. Synthesis of yttria 84Song, J., and G. Washington. 2000. Plate vibration modes identification
nanopowders for transparent yttria ceramics. Optical Materials 29(2-3): by using piezoelectric sensors. Pp. 867-878 in Smart Structures and Materi -
239-245. als 2000. International Society for Optics and Photonics 3985.

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134 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
Armor Damage Control Sensors mor designs to take advantage of the new ability to control
mechanical waves in armor materials.
The piezoelectric transducers can perform other func-
Because there are three polarizations of elastic waves
tions that contribute to armor and vehicle survivability. For
in solid materials—longitudinal, transverse (shear) in plane,
example, they can be made to act as antennas; to monitor
and transverse out of plane—for a structure to possess a
the temperature of armor structures, including that of body
full band gap, it must prohibit the propagation of all types
armor, and to detect and monitor projectile impacts on the
of waves in all directions. A phononic crystal can create an
armor surface. For the latter application, each projectile
acoustic band gap through a combination of Bragg diffrac-
striking the armor will create an electrically generated shock
tion (destructive wave interference) and Mie resonances86
wave and differing amounts of electricity; a smart sensor
as well as anticrossing of bands having the same mode sym-
can integrate these effects to generate useful information.
metry.87 Bragg scattering occurs when the wavelength of
Complex mathematical algorithms can be used to analyze
the phonon is approximately equal to the periodicity of the
the amount of electricity generated by a bullet’s impact to
structure, and Mie resonances occur when the diameter of
determine what kind of round was used, since a small-caliber
the scattering features is of the same order as the wavelength.
projectile will generate less electricity than a large-caliber
Some of the earliest references to phononic crystals are from
projectile.85 This combination of detection and assessment
Sigalas and Economou88,89 and Kushwaha et al.90,91 A rather
of threat level in real time could be significant for develop-
complete library of phononic crystal research may be found
ing armor survival strategies. The following conclusions
by consulting Vlasov and Dowling.92,93
may be drawn:
The length scale of phononic crystals ranges from the
macroscopic—meters for acoustic waves (kHz) to millime-
1. Infrared-transparent nanocomposite materials in the
ters for the ultrasound typically used in medical imaging
systems SiC/Al2O3, SiC/Si3N4, SiC/MgO, Al2O3/
(MHz), and down to the nanoregime—approximately100
ZrO2, and transparent Al2O3 offer greatly enhanced
nm for waves in the gigahertz (GHz) regime. In general,
ballistic needs for transmission in the 3 to 5 μ range,
design rules for creating a gap are based on Bragg scattering
significantly increased mechanical strength of (700
and the use of highly symmetric structures to minimize the
MPa), and fracture toughness and creep resistance
irreducible Brillouin zone over which the gap occurs. The
of interest for next-generation armor use.
materials parameters that are important are density, elastic
2. Piezoelectric transducers coupled with acoustic
modulus, and Poisson ratio or, equivalently, density and the
wave propagation and complex mathematical al-
transverse and longitudinal speed of sound in the materials.
gorithms can be used to analyze impact damage to
Because the waves are scattered from interfaces that affect
armor structures from ballistic projectiles.
wave propagation implies that understanding the differences
in mechanical impedance of the materials comprising the
PHONONIC BAND GAP CONCEPTS FOR PROTECTIVE structure (impedance is the product of density and wave
MATERIALS speed) is key to understanding how phononic crystals can
control the propagation of mechanical waves. However, in
Shortly after research on photonic band gaps began to
show promise of controlling the flow of photons, the idea
was extended to mechanical waves in periodic elastic struc- 86Mie theory, also called Lorenz-Mie theory, Lorenz-Mie-Debye theory,
tures. Referred to as phononic crystals, such structures can and Mie scattering, is an analytical solution of Maxwell’s equations for the
create what are called phononic, or acoustic, band gaps. A scattering of electromagnetic radiation by spherical particles.
87Kushwaha, M., A. Akjouj, B. Djafari-Rouhani, L. Dobrzynski, and J.
phononic crystal prevents the propagation of elastic waves
Vasseur. 1998. Acoustic spectral gaps and discrete transmission in slender
if the frequencies of the waves fall within a band gap. The tubes. Solid State Communications 106(10): 659-663.
normalized width of the band gap—the ratio of band-gap 88Sigalas, M., and E. Economou. 1992. Elastic and acoustic wave band
width to the central frequency of the gap—is a measure of structure. Journal of Sound and Vibration 158(2): 377-382.
89Sigalas, M., and E. Economou. 1993. Band structure of elastic waves
the performance of the particular phononic crystal design. In
in two dimensional systems. Solid State Communications 86(3): 141-143.
addition to preventing the propagation of waves, phononic 90Kushwaha, M., P. Halevi, L. Dobrzynski, and B. Djafari-Rouhani. 1993.
crystals dictate the nature of the modes that are allowed to Acoustic band structure of periodic elastic composites. Physical Review
propagate in the material; they can decrease the velocity Letters 71(13): 2022-2025.
of the waves and even force their negative refraction. This 91Kushwaha, M., P. Halevi, G. Martinez, L. Dobrzynski, and B. Djafari-
capability suggests numerous ideas that could someday be Rouhani. 1994. Theory of acoustic band structure of periodic elastic com -
posites. Physical Review B 49(4): 2313-2322.
developed to influence material fabrication and enhance ar- 92Yaslov, Y. Photonic band gap links. Available at http://www.pbglink.
com.
85Sands, J., C. Fountzoulas, G. Gilde, and P. Patel. 2009. Modelling 93Dowling, J. 2008. Photonic and sonic band-gap bibliography. Avail -
transparent ceramics to improve military armour. Special Issue on Transpar- able at http://phys.lsu.edu/~jdowling/pbgbib.html. Accessed on November
ent Ceramics, Journal of the European Ceramics Society 29(2): 261-266. 6, 2010.

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APPENDIX E
propagation of elastic waves is promising since in addition
solids, in addition to simple reflection at an interface, polar-
to creating a band gap, there is the possibility of creating a
ization conversion always occurs; making it difficult to form
set of band gaps that would significantly block multiple fre-
complete gaps, and a design strategy for optimal constructs
quencies. Moreover, phononic crystals permit the tailoring of
is not yet available. The use of fluids (which support only
the allowed modes and their wave speeds inside the material,
longitudinal waves) makes it difficult to form gaps because
such that the frequencies of various material loss regimes
of the conversion of shear modes into longitudinal modes at
may be matched with the density of states and frequencies
the solid-fluid interface.
of the allowed modes to provide enhanced energy absorption.
The idea of using a structured material to influence the

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